Concrete construction forms are generally prefabricated by the manufacturer for use in a particular structural application such as corner forms, linear wall forms, or form parts for assembly on the job to build structures having a curved surface and the like. Thus, in U.S. Pat. No. 2,616,148, a form assembly enables the casting of generally curved surfaces to a particular desired curvature by the use of flexible metal sheets arranged with their side portions in an overlapping relation and having reinforcing members at such side portions that are adjustably clamped to a supporting scaffold to secure the flexible sheets in the desired curved pattern to form an arched roof. Arched roof constructions are also shown in U.S. Pat. Nos. 2,436,543 and 2,933,056. U.S. Pat. No. 3,971,176 discloses a permanently formed wooden stud-truss that has a curved central arch section positioned between a pair of straight end sections. The central arch has transverse slots or notches the sides of which are moved into contact engagement to provide the desired curvature. Before being closed the slots are filled with an adhesive compound to maintain the central arch in permanent deflection.
A concrete form including a flexible panel having an adjustable curvature is described in U.S. Pat. No. 4,679,763. The concrete form is flexible to conform to any desired curvilinear shape having no radius of curvature less than the design limitations of the form. A strap or tension member substantially coextensive with the length of the form and including a plurality of elongated openings is used in association with a plurality of nut and bolt combinations to hold the form in the desired curvilinear shape. Because the single strap extended the entire length of the form, the elongated openings became quite extended at the side edges of the form. Due to the length of the form and the relatively small radii of curvatures to which the flexible form could be made to conform, a single strap could not function for both positive and negative radii of curvatures. This prior art form, accordingly, required two straps, only one of which was used in any given structure of formwork. A flexible concrete form that would reduce the number of loose parts and simplify use of the form is desired.